The present invention concerns the area of surface coatings, the said coatings being in the form of organic films. It relates specifically to a method for forming copolymer organic films by electrochemical grafting on electrically conductive or semi-conductive surfaces using aqueous solutions of precursors that are appropriately selected in order to permit the formation of these organic films in a simple and reproducible manner on the surfaces obtained by using this method, and to their applications, in particular for preparation of microelectronic components, biomedical devices or screening kits.
At the present time, several techniques exist that make it possible to apply thin organic films on substrates, each based on a family or class of adapted molecules.
The method for forming a coating using centrifuging known by the English name “spin coating” or the techniques included in the formation of coatings by immersion (“dip coating”) or deposit by vaporizing (“spray coating”) do not require any specific affinity between the molecules deposited and the substrate of interest. In fact, the cohesion of the deposited film is based essentially on the interaction between the constituents of the film, which may be, e.g., cross-linked after they are deposited to improve stability. These techniques are very versatile, applicable to all types of surfaces to be covered, and are very reproducible. However, they do not permit any effective grafting between the film and the substrate (it is a case of simple physisorption), and the thicknesses produced are difficult to control, especially for the finest deposits (less than 20 nanometers). In addition, the spin coating techniques do not allow uniform deposits unless the surface to be covered is essentially flat. The quality of the films obtained by the techniques of “spray coating” is connected with wetting of the surfaces by the powdered liquid, since the deposit does not essentially become film-like until the drops coalesce. Thus, for a given polymer, generally only one or two organic solvents exist that are capable of yielding satisfying results in terms of controlling the thickness and the homogeneity of the deposit.
Other techniques for forming an organic coating on the surface of a support, such as deposition by plasma, are described, for example, in the articles by M. Konuma, “Film deposition by plasma techniques,” (1992) Springer Verlag, Berlin; H. Biederman and Y. Osada, “Plasma polymerization processes,” 1992, Elsevier, Amsterdam, or even photochemical activation based on a similar principle: producing unstable forms of a precursor close to the surface to be covered, which evolves by forming a film on the substrate. While deposition by plasma does not require any specific properties of these precursors, photo-activation requires the use of photosensitive precursors, of which the structure evolves under the influence of luminous radiation. These techniques generally give rise to the formation of adherent films, even though it is usually impossible to discern whether this adhesion is due to cross-linking of a topologically closed film around the object or a real formation of interface bonds.
Auto-assembly of monolayers is a very simple technique to use (A. Ulman, “An introduction to ultrathin organic films from Langmuir-Blodgett films to self-assembly,” 1991, Boston, Academic Press). Still, this technique generally requires the use of molecular precursors having an adequate affinity for the surface of interest to be coated. We then speak of the precursor-surface pair such as the sulfur compounds that have an affinity for gold or silver, the tri-halogen silanes for oxides like silica or aluminum, and the polyaromatics for graphite or nanotubes of carbon. In all cases, the formation of the film is based on a specific chemical reaction between a part of the molecular precursor (the sulfur atom in the case of thiols, for example) and certain “receptor” sites on the surface. A chemisorption reaction ensures bonding. Thus, at ambient temperature and in solution, films with molecular thickness (less than 10 nm) are obtained. However, while the pairs involving oxide surfaces give rise to the formation of films that are very tightly grafted (the Si—O bond involved in the chemisorption of the tri-halogen silanes on silica is among the most stable in chemistry), this is of no use when there is an interest in metals or semi-conductors without oxide. In this case, the bond of the interface between the conductive surface and the monomolecular film is fragile. Thus, auto-assembled monolayers of thiols on gold desorb when they are heated above 60° C. or in the presence of a good solvent at ambient temperature, or even when they come in contact with a liquid oxidizing or reducing medium. In a similar manner, the Si—O—Si bonds are weakened when they are in an aqueous medium, i.e., humidity, especially with the effect of heat.
Electrografting of polymers is a technique based on initiation then polymerization by propagation of an electrically induced chain of electroactive monomers on the surface of interest, playing simultaneously the role of electrode and that of a polymerization primer (S. Palacin et al., “Molecule-to-metal bonds: electrografting polymers on conducting surfaces.,” Chem Phys Chem, 2004, 10, 1468). Electrografting requires the use of precursors that are adapted to this mechanism of initiation by reduction and propagation, generally anionic, since sometimes electrografting that is cathodically initiated is preferred, which is applicable to noble metals and non-noble metals (in contrast to electrografting by anodic polarization, which is not applicable except to noble substrates). “Depleted vinyl” molecules, i.e., carriers of functional electro-attractor groups like the acrylonitriles, the acrylates, the vinyl pyridines, etc., are especially adapted to this method, which gives rise to a number of applications in the area of microelectronics in the biomedical area. The adherence of the electrografted films is ensured by a carbon-metal type covalent bond (G. Deniau et al., “Carbon-to-metal bonds: electrochemical reduction of 2-butenenitrile,” Surface Science, 2006, 600, 675-684).
According to this electrografting technique, polymerization is indispensable for the formation of the carbon/metal interface bond: it has actually been shown (G. Deniau et al., “Coupled chemistry revisited in the tentative cathodic electropolymerization of 2-butenenitrile.,” Journal of Electroanalytical Chemistry, 1998, 451, 145-161) that the electrografting mechanism proceeds due to an electroreduction of the monomers on the surface to yield an unstable radical anion, which, if it is not present in the medium of polymerizable molecules, desorbs to return to solution (op. cit.). This desorption reaction, the addition reaction (of the Michael addition type) of the charge of the first chemisorbed radical anion on a free monomer, offers a second means for stabilizing the reaction intermediary: the product of this addition yields a radical anion again but where the charge is “at a distance from” the surface, which contributes to stabilizing the adsorbed structure. This dimeric radical anion itself may again be added to a free monomer, and thus the consequence: each new addition has additional stability by relaxation of the charge/polarized surface repulsion, which means that the interface connection of the first radical anion temporarily becomes stable to the extent that the polymerization has taken place. In other words, it has been claimed that a vinyl monomer that cannot be polymerized cannot be electro-grafted.
Among the different techniques mentioned above, electrografting is the only technique that makes it possible to produce grafted films with a specific control of the interface bond. In addition, in contrast to the plasma or photo-induced techniques, electrografting does not generate reactive species except in the immediate area of the surface of interest (in the double electrochemical layer, of which the thickness is of several nanometers in the majority of cases).
It now seems that obtaining grafted polymer films by electrografting of activated vinyl monomers on electrically conductive surfaces proceeds due to an electro-initiation of the polymerization reaction starting from the surface, followed by a growth in the chains, monomer by monomer. The reaction mechanism of electrografting has been described, in particular, in the articles of C. Bureau et al., Macromolecules, 1997, 30, 333; C. Bureau and J. Delhalle, Journal of Surface Analysis, 1999, 6(2), 159 and C. Bureau et al., Journal of Adhesion, 1996, 58, 101.
By way of example, the electrografting reaction mechanism of acrylonitrile by cathodic polarization can be represented by Diagram A following:

In this diagram, the grafting reaction corresponds to step 1, where the growth takes place starting from the surface. Step 2 is the principal parasitic reaction, which leads to obtaining a non-grafted polymer; this reaction is limited by the use of strong monomer concentrations.
The growth of the grafted chains is thus carried out by purely chemical polymerization, i.e., independently of the polarization of the conductive surface that has given rise to the grafting. This step is thus sensitive to (and is in particular interrupted by) the presence of chemical inhibitors of this growth, in particular by protons.
In Diagram A above, where electrografting of acrylonitrile using cathodic polarization is considered, the growth of the grafted chains is carried out by anionic polymerization. This growth is interrupted, in particular, by protons and it has also been demonstrated that the amount of protons makes up the major parameter that guides the formation of polymers in solution; information obtained in the course of synthesis, especially the speed of voltammograms that accompany the synthesis, show this (see in particular the article by C. Bureau, Journal of Electroanalytical Chemistry, 1999, 479, 43). Traces of water, and more generally unstable protons of protonic solvents, make up the sources of protons that are detrimental to the growth of the grafted chains.
Overall, while way to produce chemical bonds on electrically conductive or semi-conductive substrates by electrografting of different precursors using organic solutions is known, because of these reactions it remains difficult to obtain films using aqueous solutions since the adjacent reaction mechanisms (anionic-type polymerization) do not make it possible to work in water. Up to the present time, only aryl diazonium salts have made it possible to approach a solution to this problem.
Thus, as described, e.g., in the French patent application FR-A-2 804 973, electrografting of precursors such as aryl diazonium salt that carry a positive charge can be carried out due to a cleavage reaction after reduction of the cation to yield a radical that chemisorbs on the surface. Just as in electrografting of polymers, the electrografting reaction of aryl diazonium salts is electro-initiated and leads to the formation of interface chemical bonds. In contrast, to the electrografting reactions of vinyl polymers, the electrografting of aryl diazonium salts does not “need” a coupled chemical reaction to stabilize the chemisorbed species formed as a result of transfer of charge since this species is electrically neutral and not charged negatively as in the case of a vinyl monomer. It thus leads—a priori—to a stable surface/aryl group adduct.
Still, it has been shown, especially in the French patent application FR-A-2 829 046, that the aryl diazonium salts lead to very thin organic films that can grow on themselves: once the grafting on the initial surface has been carried out by electro-cleaving and chemisorption reaction, the film grows by electrical orientation reaction in such a way that a film of polymer conductor but at the cathode. This results in difficulties in carrying out a control of the organic film thicknesses resulting from electrografting of aryl diazonium salts. In summary, the combination in aqueous solution of a diazonium salt and a vinyl monomer can lead to the formation of a grafted film under the condition that the vinyl monomer is soluble in water. However, this method of obtaining grafted films is limited to the rare vinyl monomers that are soluble in water like certain acrylic acids, hydroxylated or aminated vinyls and generally leads to films of poor quality (see article by Bell and Zhang, Journal of Applied Polymer Science, 1999, 73, 2265-272).
The electro-grafting reactions that are currently available according to the prior art thus make it possible to easily obtain a certain variety of organic films on different conductive and semi-conductive substrates using organic solutions. Nevertheless, it will be necessary to extend this range in order to respond to the demands of the industry, to diversify the usage properties of such materials and thus their application possibilities. In addition, the protocols used in industry make use of organic solvents that are notoriously toxic and costly. Thus, it is also desirable to propose new procedures that are less polluting and more profitable for industries.
At the current time, no procedure exists that makes it possible to produce grafted organic films of good quality on electrically conductive or semi-conductive surfaces that can be implemented easily in a protic environment, in particular in aqueous media, using a large variety of polymerizable monomers.
It is in order to solve this technical problem that the inventors have implemented that which is the object of the invention.